In silicon nitride-ceramics, phase transformation of .alpha. to .beta. in a sintering process occurs by using powder including .alpha. type silicon nitride powder as a main component so that .beta. type silicon nitride is formed and becomes columnar crystals entangled with each other. As is well known, such an entangled structure has improved mechanical properties in comparison with a structure formed by particles having an isotropic shape. In the normal pressure sintering, the .beta. type columnar crystals entangled in three dimensions is formed. In the hot press process, the .beta. type columnar crystal is mainly oriented on a plane perpendicular to the hot pressing direction so that the mechanical properties with respect to this plane are slightly improved. However, the mechanical strength of such sintered materials is especially deteriorated at high temperature, and a sufficient fracture toughness cannot be obtained in the materials.
The silicon nitride-ceramics have problems in, especially, the deterioration in strength at high temperature and low fracture toughness.